 The goal of stratigraphy is to correlate rocks across space and time. And there are a number of different scientific questions that can be addressed by different stratigraphic techniques. And so I'm going to talk in this video a little bit about different methods of correlation as an introduction to stratigraphy. So if you remember strata are layers in rocks, in sedimentary rocks. And so stratigraphy is how you actually use those layers and correlate across them. So one of the easiest ways to do it is to use lithology, rock type. So lithostratigraphy is correlating rocks of the same type from one area to another. This is really the easiest type of stratigraphy to do because you can actually observe the rock types directly in the field. And so if I saw a red sandstone in California and I saw a red sandstone in Nevada, I could correlate them if they have the same lithology. This is particularly useful if you actually want to study aquifers, for example. There are certain types of rocks that are very good for that and they're good for it because they have the same lithology. However, just because the rock has the same lithology doesn't mean that it was deposited at the same time or is even related to each other. So for example, a sandstone in California isn't necessarily genetically related to the red sandstone in Nevada. They could have formed from completely different processes. A lot of times the question stratigraphers are asking about the rocks are related to time, which is chrono stratigraphy. And this is basically trying to correlate rocks that formed at the same time. This is much harder to do because we can't actually observe time in rocks in most cases. There are some really nice techniques we can use that we'll talk in great deal about in later videos and at the end of the quarter. One of them that we will use right now is an event. So for example, a volcanic eruption, if it produces ash that gets preserved on the sedimentary record and we can correlate that ash bed across places, then that gives us sort of a moment in time. Other events could be something like a meteorite impact, tsunamis that affect an area, things like that. So event stratigraphy is a subset of chrono stratigraphy. We also have biology and evolution. So we can use the distribution of fossils as another form of subset of chrono stratigraphy or bio stratigraphy. The Earth's magnetic field changes and when it reverses direction, that happens across Earth, nearly geologically simultaneously, so magneto stratigraphy is another subset of chrono stratigraphy. Then we can also use sea level changes, which are really useful and we actually call the sequence stratigraphy. And this is one of the most powerful tools for correlating environments through time. It uses Walther's law and concepts of stratigraphy to be able to reconstruct large parts of Earth history. So the next set of videos I'm going to describe and demonstrate some of the differences between litho stratigraphy and chrono stratigraphy. Thanks for watching.